As class AB amplifiers have good audio performance and they are widely used in audio devices. However, the power consumption of class AB amplifiers is so large that they may not be used in portable devices which are powered by batteries.
Class D amplifiers are another type amplifier which have good audio performance and consume less power. In operation, class D amplifiers convert an analog signal to a digital signal (e.g., pulses) and then convert the digital signal back into an amplified analog signal. In the process of this signal conversion, many kinds of disturbances may affect the output amplified analog signal from class D amplifiers, and these disturbances may make the audio uncomfortable for the listener.
During the operation of class D amplifiers, audible pop noises are generated in response to transient events. If a large sound signal starts/finishes suddenly or the audio device including the class D amplifier stops/starts suddenly, a pop noise is generated in correlation with these events and such pop noise is easily heard by the listener.
Because of the steps of fabrication process, both offset of input stages of the class D amplifiers and mismatching of resistors in the feedback loop may contribute to offset in output. Without any suppression, this offset (especially when this offset is larger than 20 mV) produces an audible pop noise at transient events where the output stage starts to switch on or switch off. If a large audio signal starts from a silent environment or a large audio signal stops suddenly, the resulting pop noise is uncomfortable for the listener. This event is one kind of pop noise.